1. Field of the Invention
The present invention relates to an electrochemical cell and process for converting essentially anhydrous hydrogen halide to essentially dry halogen gas. The process of the present invention is useful for converting anhydrous hydrogen halide, in particular, hydrogen chloride, hydrogen fluoride, hydrogen bromide and hydrogen iodide, to a halogen gas, such as chlorine, fluorine, bromine, or iodine. In particular, in the present invention, the electrochemical cell has a mass flow field that increases the diffusion resistance of a fluid within either a cathode or an anode compartment of the cell.
2. Description of the Related Art
Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinyl chloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride or the acid produced often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product.
A number of commercial processes have been developed to convert HCl into usable chlorine gas. See, e.g., F. R. Minz, "HCl-Electrolysis--Technology for Recycling Chlorine", Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK, 4/21-4/23, 1993.
Currently, thermal catalytic oxidation processes exist for converting anhydrous HCl and aqueous HCl into chlorine. Commercial processes, known as the "Shell-Chlor", the "Kel-Chlor" and the MT-Chlor" processes, are based on the Deacon reaction. The original Deacon reaction as developed in the 1870's made use of a fluidized bed containing a copper chloride salt which acted as the catalyst. The Deacon reaction is generally expressed as follows: ##STR1## where the following catalysts may be used, depending on the reaction or process in which equation (1) is used.
______________________________________ Catalyst Reaction or Process ______________________________________ Cu Deacon Cu, Rare Earth, Alkali Shell-Chlor NO.sub.2, NOHSO.sub.4 Kel-Chlor Cr.sub.m O.sub.n MT-Chlor ______________________________________
The commercial improvements to the Deacon reaction have used other catalysts in addition to or in place of the copper used in the Deacon reaction, such as rare earth compounds, various forms of nitrogen oxide, and chromium oxide, in order to improve the rate of conversion, to reduce the energy input and to reduce the corrosive effects on the processing equipment produced by harsh chemical reaction conditions. However, in general, these thermal catalytic oxidation processes are complicated because they require separating the different reaction components in order to achieve product purity. They also involve the production of highly corrosive intermediates, which necessitates expensive construction materials for the reaction systems. Moreover, these thermal catalytic oxidation processes are operated at elevated temperatures of 250.degree. C. and above.
Electrochemical processes exist for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution. The current electrochemical commercial process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22% is fed at 65.degree. to 80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17% with the production of chlorine gas and hydrogen gas. A polymeric separator divides the two compartments. The process requires recycling of dilute (17%) HCl solution produced during the electrolysis step and regenerating an HCl solution of 22% for feed to the electrochemical cell. The overall reaction of the Uhde process is expressed by the equation: ##STR2## As is apparent from equation (2), the chlorine gas produced by the Uhde process is wet, usually containing about 1% to 2% water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. This possible side reaction of the Uhde process due to the presence of water, is expressed by the equation: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- ( 3)
Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps/ft.sup.2, because of this side reaction. The side reaction results in reduced electrical efficiency and corrosion of the cell components.
Another electrochemical process for processing aqueous HCl has been described in U.S. Pat. No. 4,311,568 to Balko. Balko employs an electrolytic cell having a solid polymer electrolyte membrane. Hydrogen chloride, in the form of hydrogen ions and chloride ions in aqueous solution, is introduced into an electrolytic cell. The solid polymer electrolyte membrane is bonded to the anode to permit transport from the anode surface into the membrane. In Balko, controlling and minimizing the oxygen evolution side reaction is an important consideration. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell. The design and configuration of the anode pore size and electrode thickness employed by Balko maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from FIGS. 3 to 5 of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at high current densities, the anode would be destroyed.
In an electrochemical cell, the conductivity of a membrane is directly related to the water content in the membrane and decreases at low water content. Limiting current density occurs when the concentration of water within the membrane reaches a value that will no longer support additional proton conduction. Therefore, limiting current density can develop when the conductivity decreases due to low water concentrations. It is important to regulate limiting current so that the components of the cell are not destroyed.
Use of a thin, foraminous flexible sheet such as fine mesh stainless steel, nickel screen, stainless steel or graphite paper interposed between an electrode and a compressible current collector in an electrolytic cell which processes an aqueous sodium chloride solution is disclosed in U.S. Pat. No. 4,343,690 to de Nora. The purpose of this sheet is to protect the electrode from the collector. It also provides a greater total contact area than would be provided by the current collector alone, since the current collector in this patent, being an open mesh metal wire screen, is more open in its construction than known current collectors. In this de Nora patent, there is no recognition of a desire to increase the diffusion resistance of the fluid in the electrode compartment.
There exists a need for directly producing essentially dry halogen gas without having to first dissolve the hydrogen halide in water, and for decreasing the diffusion resistance of water in the membrane during such a process. This would allow the limiting current density of the cell to be increased and/or controlled, so that the cell components would not be destroyed. It would be especially desirable to provide a self-regulating mechanism within a cell which would internally control limiting current, without having to externally adjust the parameters of the cell.